1. Field of the Invention
The present invention is in the field of prescription ophthalmic lenses and particularly relates to a method for making colored or tinted photochromic prescription lenses having prescribed combinations of light-filtering and light-refracting properties.
2. Description of the Prior Art
The use of photochromic glass for brightness-compensating sunglasses and prescription ophthalmic lenses is well known. Photochromic glasses, which are glasses containing silver halide crystallites which reversibly darken on exposure to light and fade upon removal of the light, were first described in U.S. Pat. No. 3,208,860. A number of more advanced photochromic glasses have since been developed and described in the patent literature, including U.S. Pat. No. 4,190,451 which describes some fast-fading photochromic glasses exhibiting desirably low interdependence between photochromic darkening and glass temperature.
The adjustment of the undarkened transmittance of photochromic glasses by tinting, for cosmetic purposes, eyewear comfort and for medical reasons, has also been of interest. German utility model Gm No. 1,985,755 describes configurations for laminated photochromic ophthalmic lenses wherein photochromic glass lens elements are cemented to tinted glass or thin film/glass backing elements to introduce fixed tints to prescription or non-prescription lenses. It is also known to introduce colorants such as NiO, Cr.sub.2 O.sub.3, CuO, V.sub.2 O.sub.5, MnO, Pr.sub.2 O.sub.3, and Er.sub.2 O.sub.3 into photochromic glasses to modify the undarkened transmittance thereof.
Following the development of photochromic glasses it was reported that colored surface layers could be developed in silver-halide containing glasses by heat treating the glasses under reducing conditions. Hence U.S. Pat. Nos. 3,892,582 and 3,920,463 disclose the production of photochromic glasses exhibiting yellow colors, while U.S. Pat. Nos. 4,240,836, 4,284,686, and 4,290,794 disclose further heat treatment methods for generating other surface colors in silver-halide-containing photochromic glasses.
Adding a coloring heat treatment to the existing commerical process for producing prescription photochromic ophthalmic lenses has a significant complicating effect on the process, particularly since the process already involves multiple steps which have to be performed in a specific sequence. Hence, in addition to the heat treatment required to develop photochromic properties in the lens blanks and the heat treatment required to develop coloration in the lenses, the further steps of prescription grinding of the lenses, chemical or thermal strengthening of the lenses to meet U.S. Food and Drug Administration strength requirements, and edging of the lenses for framing are required.
The latter three steps of the above-described process are conveniently carried out in small optical laboratories close to the point of lens purchase. However, the coloring operation requires careful process control in order to provide reproducible tints (lenses of the same nominal tint must be sufficiently close in color to be interchangeable), and is best carried out on a large scale under factory conditions, preferably as an adjunct to the lens manufacturing process.
Unfortunately, the coloring process can only be applied to finished lens surfaces, since the grinding and polishing of a tinted surface will remove or modify the color in an uncontrollable way. Further, in many cases the color should be present only on the back or prescription ground portion of the lens in order to avoid undue light absorption by the front surface, with its attendant loss of photochromic response characteristics. In commercial practice this has necessitated the return of finished or semifinished lenses from the optical laboratory to the lens manufacturer for coloring, causing undue delays in the filling of ophthalmic prescriptions.
It is therefore a principal object of the present invention to provide an alternative and improved process for the manufacture of colored photochromic ophthalmic lenses which provides a reproducible product of high quality more quickly and at reduced expense.
Other objects and advantages of the invention will become apparent from the following description thereof.